This invention relates to furnaces for melting thermoplastic materials, such as glass, and more particularly to those furnaces having a protective liner within the melting chamber of the furnace. As pointed out in U.S. Pat. No. 4,366,571, the use of a refractory metal liner materially extends the useful life of a melting furnace by protecting the refractory sidewalls and bottom walls of the melting chamber from the corrosive action of the molten bath contained therein.
In standard refractory glass melting furnaces, bottom electrodes are generally utilized to assist in the start up of the furnace and to maintain the temperature of the melt adjacent the bottom at a desired thermal level. Since the refractory, forming the vessel of a standard furnace, is usually non-conductive and functions as an electric isolator, there has been no difficulty with respect to electrical shorting when immersing electrodes through the lower portion of the sidewalls or up through the bottom walls of the furnace. However, when a furnace is lined with a refractory metal protective liner, wherein a molybdenum or other refractory metal bottom plate overlies the refractory bottom of the furnace, there is no possibility of immersing the electrodes through the molybdenum lining, since such lining would short circuit the electrical supply to the electrodes.
In view of the fact that it is not feasible to immerse electrodes up through the bottom of a furnace having a protective metal liner, it became necessary to supply bottom heat to the furnace by positioning resistance heaters in the space between the bottom wall and the bottom liner. As shown in the prior art furnace 10, of FIG. 1, a plurality of resistance heaters 32 are positioned within the limited space provided between the protective liner and the refractory walls of the vessel. That is, the furnace 10 includes a refractory vessel 12 having upstanding sidewall portions 14 and a bottom wall portion 16. The refractory vessel 12 is surrounded by an outer shell 18 and may have insulation 20 therebetween.
A refractory metal liner 22 is positioned within the vessel 12 in closely spaced-apart relationship therewith such that the sidewall portions 24 of the liner are spaced-apart from the upstanding portions 14 of the refractory vessel with a limited cavity or space 25 therebetween. The bottom wall portion 26 of the liner is spaced-apart from the bottom wall 16 of the refractory vessel by means of spacers 28, forming a cavity or space 30 between the bottom wall 16 of the refractory vessel and the bottom wall 26 of the protective liner. As shown, a plurality of resistance heaters 32 are positioned within the space 25 behind the sidewall 24 of the liner and within the space 30 below the bottom wall 26 of the liner.
The utilization of such resistance heaters has not been completely satisfactory due to installation problems and inefficient operations. The sidewall heaters, for example, are different to install in view of the fact that installation involves the removal of insulation from around the side of the furnace, adding to the complexity and cost of installation. In addition, the gap between the liner and the refractory adjacent the bottom of the vessel is filled with a noncorrosive glass that may have zirconia particles suspended in a silica matrix, and has a very low conductivity. Accordingly, any resistive heater placed behind the liner has a tendency to overheat, since the protective glass is an insulating material.
In a like manner, for the wire mesh resistive heaters 32, lying on top of the bottom refractory wall 16 underneath the bottom plate 26, to transmit energy to molten glass 34 within the vessel 22, the heaters must first transmit energy through a layer of glass or tamp within the space 30 and then through the bottom plate 26 of the liner 22. Accordingly, the temperature of the resistance heater tends to become hotter in order to transmit the energy to the bottom plate, which results in the refractory therebeneath to disassociate or becomes soft, thus leading to the failure of the resistance heater. To restate the problem, the high temperature of the resistance heater softens the refractory bottom wall 16, upon which it is resting, so that the resistance heater 32 tends to sink into the bottom refractory and finally breaks or burns out due to its high temperature.
In U.S. Pat. No. 2,781,411, molten glass within a crucible may be provided with Joule heating by means of electrodes positioned within the crucible or by utilizing the crucible per se as an electrode. In addition, a resistance heater in the form of a heating spiral, may be positioned externally of the crucible for supplementing the heat supplied by the electrodes.
In U.S. Pat. No. 3,109,045, the melting of the glass within the melting pot is accomplished through the resistance heating of the pot per se. In addition, molten glass surrounding the pot is heated by electrodes, whose circuits may be completed through the melting pot per se.
The utilization of bottom heaters in furnaces having refractory metal liners is more crucial to the operation of the furnace than in standard refractory furnaces, since metal lined furnaces require that the operating energy be provided by electrodes immersed through a batch blanket which rests upon the surface of the molten bath. Therefore, when there is an interruption in the supply of energy from such electrodes, such as during start-up wherein an initial protective crown is removed from the furnace after a batch blanket is formed, or during a malfunction of the batch electrodes, the bottom heating means would function as the only means to maintain the bath in its molten state, until such time as repairs could be completed.
The present invention overcomes the problems encountered with the known prior art furnaces by providing a system of bottom heaters which is not unduly influenced by the glass mixture beneath the bottom liner, and which more effectively transfers heat to the glass within the molten bath without deleteriously affecting the refractory material adjacent the heater.